Speciation of five arsenic species (arsenite, arsenate, MMAA
V
,
DMAA
V
and AsBet) in different kind of water by HPLC-ICP-MS
Se´bastien N. Ronkart
a,*, Vincent Laurent
b,1, Philippe Carbonnelle
b,1,
Nicolas Mabon
a,2, Alfred Copin
a,2, Jean-Paul Barthe´lemy
a,2aGembloux Agricultural University, Unite´ de chimie analytique et Phytopharmacie, Passage des De´porte´s, 2, B-5030 Gembloux, Belgium bLaboratoire Central de la Socie´te´ Wallonne Des Eaux, Avenue de l’Espe´rance, B-6220 Fleurus, Belgium
Received 22 January 2006; received in revised form 19 July 2006; accepted 20 July 2006 Available online 7 September 2006
Abstract
A method using Ion Chromatography hyphenated to an Inductively Coupled Plasma-Mass Spectrometer has been developed to accu-rately determine arsenite (AsIII), arsenate (AsV), mono-methylarsonic acid (MMAAV), dimethylarsinic acid (DMAAV) and
arsenobe-taine (AsBet) in different water matrices. The developed method showed a high sensitivity with detection limits for each arsenic species close to 0.4 pg injected. Arsenite and arsenate were the major species found in surface and well waters, but AsBet and DMAAV were found in some surface waters, which has never been reported before, while in some natural mineral waters located in volcanic region, the arsenic content exceeded the maximal admissible arsenic content by European legislation standards and the predominant form was AsV.
2006 Elsevier Ltd. All rights reserved.
Keywords: Arsenic; IC-ICP-MS; Speciation; Trace analysis
1. Introduction
Arsenic exists under the form of various chemical spe-cies differing, not only by their physicochemical behaviour, but also in toxicity, bioavailability and biotransformation (Maeda, 1994). With more than 20 arsenic compounds present in the natural environment and biological systems, it is important to identify each individual chemical species of the element in the samples (Le et al., 2004). For this rea-son, the notion of speciation, defined as the determination of all the individual physicochemical forms of an element, has gained interest in recent years.
Arsenite (AsIII) and arsenate (AsV) are the most toxic forms, considered and classified as human carcinogen sub-stances and are the predominant forms found in water. Biologically-mediated methylation reactions, occurring in terrestrial and marine organisms, convert arsenite and arse-nate to methylated compounds of moderate toxicity, such
as methylarsonic acid (MMAAV) and dimethylarsinic acid
(DMAAV). Arsenobetaine (AsBet), a more complex
arsenic compound, was identified essentially in marine biota (Goessler et al., 1998a,b; Ackley et al., 1999; Kohl-meyer et al., 2002) or in marine sediments (Takeuchi et al., 2005) and is considered to be relatively non-toxic (Kaise et al., 1985).
The admissible level of arsenic in natural mineral water (Commission Directive 2003/40/CE of 16 May 2003) and in water intended for human consumption (Commission Directives 98/83/CE of 3 November 1998), has been
low-ered to 10 lg l1 of total arsenic. However, as reported
previously in literature (Van Holderbeke et al., 1999), some
0045-6535/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.07.056
*
Corresponding author. Tel.: +32 8162 2111; fax: +32 8162 2216. E-mail addresses:ronkart.s@fsagx.ac.be(S.N. Ronkart),labo@swde. be(V. Laurent).
1 Tel.: +32 7182 5911; fax: +32 7182 5900. 2 Tel.: +32 8162 2248; fax: +32 8162 2216.
commercially available natural mineral water exceeded this limit of 10 lg l1.
In order to assess the individual forms of arsenic, which may be present in water, and thus the potential toxicity, the analytical equipment must achieved very low detection limits.
Hyphenated analytical techniques are often necessary to achieve both selectivity and sensitivity for arsenic specia-tion at a low concentraspecia-tion level. Numerous instrumental methods for these five arsenic species speciation are reported in the literature. Most of them are based on chro-matographic separation techniques such as High
Perfor-mance Liquid Chromatography (HPLC) (Gailer and
Irgolic, 1996; Tera¨sahde et al., 1996; Le and Ma, 1997; Dagnac et al., 1999; Kohlmeyer et al., 2002) or Capillary
Zone Electrophoresis (Van Holderbeke et al., 1999),
cou-pled with a selective and sensitive detector such as Atomic
Absorption Spectrometry (AAS) (Gailer and Irgolic, 1996)
or Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (Tera¨sahde et al., 1996; Kohlmeyer et al., 2002). Post column derivatization of arsenic species with formation of the volatile arsine hydrides, has been coupled with AAS (Le and Ma, 1997), Atomic Fluorescence Spectrometry (Simon et al., 2004) or ICP-MS (Dagnac et al., 1999), but cannot be used for quantifying AsBet without prior chem-ical conversion (Slejkovec et al., 1999).
The hyphenated HPLC-ICP-MS technique is the most powerful method for arsenic speciation, allowing detection
limits generally lower than 0.5 lg l1 (B’Hymer and
Car-uso, 2004, and references therein). The advantages associ-ated with the HPLC-ICP-MS technique include high elemental specificity, the possibility to record real time chromatograms, the ability to separate the signals of inter-fering species from the peaks of interest, a high linear range, a multi-element capability and the possibility of iso-topic information. The high sensitivity of ICP-MS allows the analysis of water samples without time-consuming pre-concentration and derivatization steps. This consideration is crucial as species may be converted from one form to another or lost during the pre-treatment of the sample (Gong et al., 2002). However, the use of ICP-MS as a detector for HPLC gives rise to some constraints on the choice of chromatographic conditions concerning the nat-ure and concentration of the buffer salts of the mobile phase and the presence of organic solvents. Moreover, because of its high sensibility, ICP-MS may suffer from many isobaric interferences caused mainly by phenomena occurring either in the plasma or in the ion extraction device. For example, presence of chlorine in the sample may give rise to the formation of40Ar35Cl+ that interferes strongly with the mono-isotopic75As+(Gray, 1986; Hywel Evans and Giglio, 1993).
For these reasons, the aim of this work was to develop a powerful speciation method suitable for trace analysis of all arsenic species found in drinking and surface water with appropriate performance characteristics in order to identify and quantify each arsenic species.
2. Experimental
2.1. Standards and reagents
Stock solutions of arsenic species (1000 mg l1) were pre-pared from NaAsO2 (arsenite, AsIII), Na2HAsO4Æ 7H2O
(arsenate, AsV), (CH3)2AsO(OH) (dimethylarsinic acid,
DMAAV), CH3AsO(OH)2 (mono-methylarsonic acid,
MMAAV) and (CH3)3As+CH2COO (arsenobetain,
AsBet). AsIII, AsV and DMAAV were obtained from
Sigma–Aldrich (Steinheim, Germany) while MMAAVand
AsBet were obtained from Argus Chemicals (Florence, Italy). These individual standards were checked for purity by IC-ICP-MS, and the stock solutions were stored at 4C before analysis. In these conditions, the stock solutions were stable for at least one month. Analytical working solu-tions were prepared by diluting the stock solusolu-tions with
ultrapure water (18.2 MX cm1, Millipore, Molsheim,
France) prior to analysis.
For the chromatographic mobile phase, NH4H2PO4
(ammonium dihydrogen phosphate, >99%, Sigma–Aldrich,
Steinheim, Germany), NH4OH (ammoniac 25%,
Supra-pur, Merck, Darmstadt, Germany) and methanol
(Supra-solv, Merck, Darmstadt, Germany) were used.
2.2. Instrumentation
The High Performance Liquid Chromatography
(HPLC) module consisted of an Agilent 1100 Series (Agi-lent Technologies, Yokogawa Analytical System Inc., Tokyo, Japan) with a handheld control module. The sepa-ration of the arsenic species was performed on a Dionex
AS7 anion-exchange column (250· 4 mm; 10 lm) bearing
alkyl quaternary ammonium exchange sites on a styrene-divinylbenzene copolymer. 20 ll of the sample was injected in the chromatographic column.
The Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) system consisted of an ICP-MS Hewlett-Packard 4500 Series 200 (Agilent Technologies, Yokogawa
Analyti-cal System Inc., Tokyo, Japan). A 40 cm PEEK tubing
(PolyEther Ether Ketone, 1/1600 OD X 0.01000 ID
Blue-Stripe) with appropriate fittings was used to connect the outlet of the HPLC analytical column directly to the inlet
of the ICP concentric nebulizer (Conikal Concentric
Nebulizer-STF, Glass Expansion, Switzerland). A double-pass spray chamber and a quartz torch were used in this study. The temperature of the spray chamber was
main-tained at 2C by cooling Peltier Effect. In accordance with
B’Hymer et al. (1998), this concentric nebulizer/double pass spray chamber combination allows the best signal stability. In addition to the ICP-MS Hewlett-Packard 4500 Series 200 (only used for the coupling with HPLC), an ICP-MS Hewlett-Packard 4500 Series 100 (Agilent Technologies, Yokogawa Analytical System Inc., Tokyo, Japan) was used for the total arsenic concentration in water. The ICP-MS Hewlett-Packard 4500 Series 100 was equipped with a Babington nebulizer.
2.3. Tuning and data acquisition
Before all analyses, the instrument was tuned using a 10 lg l1 lithium (7Li), yttrium (89Y), thallium (205Tl) and
cerium (140Ce) solution (all Certipur, obtained from
Merck, Darmstadt, Germany). Resolution and mass axis were optimized by monitoring m/z7Li,89Y and205Tl.
Sen-sitivity was maximized at m/z 89Y, allowing a very high
sensitivity for 75As, and thus very low detection limits,
while maintaining the ratio of oxides (140Ce16O/140Ce) and doubly charged ions (140Ce2+/140Ce+) at a low level to minimize the potential interferences.
For IC-ICP-MS data acquisition, the time resolved analysis mode was used. In addition of the arsenic signal at m/z 75, the interferences from chloride were also checked by monitoring m/z 35, 77, 82 and 83. The quantification of the chromatographic peaks was based on the peak area. 2.4. Speciation of arsenic in water
Surface, well and natural mineral water samples were analysed by IC-ICP-MS. These samples were collected from several locations in the Walloon Region (Belgium)
and stored in polyethylene flasks at 4C without
acidificat-ion to prevent changes in species distributacidificat-ion. The analyses were carried out within one week. Natural mineral waters were obtained from a local market, and covering different geological origins. The sum of arsenic species concentra-tions was compared to the total arsenic content obtained by ICP-MS analysis. All real samples were filtered through
5, 0.45 and 0.2 lm filter membranes (all Acrodisc25 mm
Syringe filter with Versapormembrane, Gelman Sciences,
MI, USA) directly into the auto sampler vial and injected in the chromatographic system. All samples were analysed in triplicate.
3. Results and discussion
The optimized chromatographic conditions and the instrumental parameters used for IC-ICP-MS are summa-rized inTable 1.
3.1. Set up of the chromatographic elution
In HPLC, Ion Chromatography (IC) is an attractive technique for elemental speciation because it can separate charged species. Except AsBet, which is a zwitterion, all the other arsenic species of this study have a range of disso-ciation constants making them suitable for anion exchange column, as they exist in anionic form in alkaline mobile phase (Tera¨sahde et al., 1996). Na2HPO4 and NaH2PO4
are often used as mobile phase for the arsenic species sepa-ration, but deposition of salt on the sampling interface causes a rapid degradation and instability of the signal. For this reason, the selected mobile phase used in this study
was ammonium dihydrogen phosphate (NH4H2PO4), less
deposit was observed together with a good stability of the signal. In the investigated NH4H2PO4concentration range
(1–50 mM), the order of elution was AsBet, DMAAV, AsIII, MMAAVand AsV(Fig. 1). NH4H2PO45 mM gave the best
compromise between a good AsBet/DMAAV resolution
and minimum tailing for AsIII. Furthermore, the effect of
the pH of the mobile phase was tested (7.0, 8.0, 9.0 and
10.0) by adjusting with NH4OH 1.35 M. Increasing the
Table 1
Instrumental settings Total arsenic
ICP-MS HP 4500 Series 100 Argon plasma gas flow rate (l min1) 15
Argon carrier gas flow rate (l min1) 1
Argon auxiliary gas flow rate (l min1) 1
Peristaltic pump flow rate (l min1) 1
Power (W) 1400
Arsenic speciation
HPLC Agilent 1100 Series
Analytical column Dionex IonPak AS7 (250· 4 mm; 10 lm) Mobile phase A 2.5 mM NH4H2PO4pH 10.0 (NH4OH)
Mobile phase B 50 mM NH4H2PO4
Gradient program Time (min) A (%) B (%)
0–4 100 0
15–20 0 100
24–30 100 0
ICP-MS HP 4500 Series 200 Argon plasma gas flow rate (l min1) 15
Argon carrier gas flow rate (l min1) 0.7 Argon auxiliary gas flow rate (l min1) 1.0 Argon blend gas flow rate (l min1) 0.5
Power (W) 1400
pH of the mobile phase led to a higher ionisation degree of
arsenic compounds according to their pKa and should
induced a higher affinity for the available exchange sites on the column and thus their retention time. But, the disso-ciation of the phosphate ions of the buffer, and thus the elu-ent power of the mobile phase, also increased with the pH. The retention time obtained as a function of the pH is the
result of these antagonist phenomenons. For this reason,
a 2.5 mM NH4H2PO4 solution adjusted to pH 10.0 with
NH4OH allowing a good AsBet/DMAA
V
separation together with a minimum tailing for arsenite, was selected. This mobile phase was suitable for the separation of AsBet,
DMAAV and AsIII, but not for MMAAV and AsVdue to
the very long retention time. Consequently, an elution
gra-dient with NH4H2PO4 50 mM was used. This gradient
allowed a correct separation of the five molecules within 30 min including column re-equilibration time. In order to improve the ionisation of arsenic in the plasma, 3% (v/v)
of methanol have been added in the mobile phase (
Beauche-min et al., 1989; Larsen and Stu¨rup, 1994). In spite of the fact thatBrisbin et al. (2002)claimed that the use of carbon-ate as mobile phase brings less deposit than phosphcarbon-ate, no signal decrease was observed with ten repetitions. The rela-tive standard deviations of the peak area were 1.5, 1.5, 2.0,
0.8 and 1.6% for AsBet, DMAAV, AsIII, MMAAV, and
AsV, respectively. The stability of the signal and the low standard deviation avoid the use of an internal standard. 3.2. Validation of the method
For the applicability of a method to real samples, a val-idation study is indispensable. Thus, the performance of the method was estimated by determining the limit of detection (LOD), the limit of quantification (LOQ), the linearity and the precision (within-assay and between-assay precision) of results following the XP T 90-210 (1994) guideline and cal-culations according to ISO 5725 (1996) (Table 2).
The linearity was checked in the concentration range 0.1– 50 lg l1 for each arsenic species by calculating the coeffi-cient of determination (R2) of the linear regression based on six different concentrations (three replicates) including the blank. The linear regression equations were y =
17.237x 0.8008; y = 18.223x 0.5866; y = 20.172x
2.6757; y = 19.09x + 1.4758 and y = 18.564x + 10.1 for
respectively AsBet, DMAAV, AsIII, MMAAV and AsV. R2
obtained after linear regression were in all cases better than 0.9999, clearly illustrating adequate linearity for all the five arsenic species in the concentration range investigated.
Precision was expressed by within-assay precision (WAP) and between-assay precision (BAP) and were calcu-lated according to Eqs. (1) and (2),
Fig. 1. Anion exchange HPLC-ICP-MS chromatogram of a 20 ll injected standard solution containing 5 lg l1 of each arsenic species. Peak identification: (1) AsBet, (2) DMAAV, (3) AsIII, (4) MMAAV and (5) AsV. The analysis was performed according to the optimum conditions
shown inTable 1.
Table 2
Within-assay precision (WAP), between-assay precision (BAP), limit of detection (LOD) and limit of quantification (LOQ) of the method using conditions inTable 1
Arsenic speciesb WAPa BAPa(%) LOD LOQ
0.1 10 50 0.1 10 50 Relative (ng l1) Absolute (pg) Relative (ng l1) Absolute (pg) AsBet 2.4 1.0 3.0 3.4 1.0 3.7 24 0.5 80 1.6 DMAAV 3.3 1.0 2.6 3.5 1.0 3.8 23 0.5 76 1.5 AsIII 4.0 1.4 2.2 6.8 5.9 2.6 17 0.3 56 1.1 MMAAV 4.3 1.4 3.7 4.3 2.2 3.8 26 0.5 88 1.8 AsV 5.7 1.4 4.0 6.2 1.7 4.6 26 0.5 85 1.7 a
Concentrations used for calculating these parameters are expressed in lg l1.
b
WAP¼100 m ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pk j¼1 ðnj 1Þ S2j Pk j¼1 nj ! k v u u u u u u u t ð1Þ BAP¼100 m n Pk j¼1 ½nj ðmj mÞ2 k 1 ð2Þ
where m is the used concentration; S2
j is the intergroup
var-iance; nj is the number of the group j repetition; k is the
number of the group (k = 4); mj is the average of the j
group; m is the average mj and n is the average number
of values per group. WAP and BAP were determined at
0.1, 10 and 50 lg l1 with four analysis groups. The first
group included 10 repetitions, while for the three others, only six replicates were carried out. At concentrations close to the LOQ, WAPs were <6%, showing good precision of the obtained results for trace arsenic analysis. Moreover, BAPs were similar to WAPs, proving low variation of the results in the course of time. Limit of detection and quan-tification (LOD and LOQ) were evaluated by analysing ten samples containing an arsenical concentration close to the expected LOD and LOQ in repeatability conditions and using Eqs.(3) and (4),
LOD¼ ysignalþ 5 Ssignal ð3Þ
LOQ¼ ysignalþ 10 Ssignal ð4Þ
where ysignalis the medium value and Ssignalis the standard
deviation.
LODs were closely the same for all the compounds and made the method particularly suitable for trace arsenic spe-cies determination in real water samples and were better than detection limits previously reported in the literature.
LOQ values were confirmed by analysing ten replicates of a solution containing all the arsenic species at the LOQ. Variation coefficient of 9.87, 8.38, 8.33, 12.3 and
8.84% were obtained for respectively AsBet, DMAAV,
AsIII, MMAAVand AsV.
3.3. Arsenic analysis in reference materials
Certified reference materials for arsenic speciation were not available for drinking water. Therefore,
interlabora-tory solutions with well established concentrations for total
arsenic were selected, namely Aquacheck (Aquacheck
Ltd, Bury Greater Manchester, England).
The certified total arsenic concentration agrees closely
with the speciation results (Table 3). However,
simulta-neous presence of AsIIIand AsVwas also noticed, probably
Table 3
Speciation of the standard material water and comparison with the total arsenic expecting concentration
Water samples IC-ICP-MSb Expecting value AsBet DMAAV AsIII MMAAV AsV Total
Aquackeck 1 a a 4.44 ± 0.36 a 1.98 ± 0.20 6.42 ± 0.41 6.50 ± 1.10 Aquackeck 2 a a 1.88 ± 0.21 a 1.20 ± 0.13 3.10 ± 0.25 2.91 ± 0.49
Aquackeck 3 a a 2.45 ± 0.26 a 1.25 ± 0.14 3.70 ± 0.29 3.53 ± 0.60
Aquackeck 4 a a 4.73 ± 0.37 a 1.55 ± 0.16 6.28 ± 0.40 6.78 ± 1.15
The values are expressed in lg l1. a <LOD.
bAsBet (arsenobetaine), DMAAV(dimethylarsinic acid), AsIII(arsenite), MMAAV(methylarsonic acid), AsV(arsenate).
Fig. 2. Anion exchange HPLC-ICP-MS chromatogram of a 20 ll injected standard solution containing 10 lg l1of AsVand 200 mg l1 of NaCl. Peak identification: (1) Chloride and (2) AsV. The analysis was performed according to the optimum conditions shown inTable 1.
resulting from the oxidation of the trivalent to the penta-valent form. Indeed, these solutions were prepared with an arsenite form and this conversion can be related to the
2% Aquacheck HNO3 acidification and prove the
arsenic-form stabilisation problem in water. This
hypothe-sis was confirmed byBohari et al. (2001)who showed that
0.5% HNO3acidification changed arsenic speciation after
ten days of storage. Thus, it will be important to consider this aspect for the preparation of certified water samples containing arsenic species, as this kind of sample does not exist yet.
3.4. Arsenic speciation in real samples
The possible interference of40Ar35Cl+ with the mono-isotopic75As+, has been studied by analysing solutions con-taining increased concentration (200, 500 and 1000 mg l1) of chloride and comparing the signal obtained at m/z 75 with the signal of a blank (Milli-Q water) using a t-test. This test was of particular importance since chloride eluted at the
same retention time as arsenate (Fig. 2). We found that
there is no noticeable 40Ar35Cl+ interference up to
500 mg l1 chloride. These results are in good agreement
with those published bySaverwyns et al. (1997) and
Pant-sar-Kallio and Manninen (1997).
Possible matrix effects on the calibration were estimated by spiking representative real water samples. These samples were collected at various locations in order to have differ-ent matrix contdiffer-ents, e.g. high suspended colloids. Samples from ground, well and surface water were spiked with a standard mixture of the five arsenic species giving an added
arsenic concentration of 0.5 and 5 lg l1 each. When the
IC-ICP-MS procedure was applied to the analysis of three spiked real water samples, recoveries were satisfactory with values ranging from 95% to 108%. Then, surface, well and natural mineral waters were analysed for the native arsenic species quantification (Table 4).
AsIII and AsV were the major arsenic species found in
surface and well waters, but AsBet and DMAAVwere
pres-ent only in surface water. To our knowledge, it was the first time AsBet was found in surface waters, but the detection of this compound can be related to the fact that the proposed method was characterized by the lowest LOD in literature, permitting an AsBet ultra trace quantification. The presence of such methylated compounds probably results from a more intense biological activity in this kind of environment, as the bioaccumulation of the methylated forms has been largely discussed in terrestrial and marine organisms.
More-over,Hanaoka et al., 1997found AsBet in suspended
parti-cles in marine biota (probably from degraded organisms),
while Takeuchi et al. (2005) claimed an incorporation of
this arsenic specie into the sediment. They quantified AsBet
and DMAAV as the major compounds in the sediment,
although the concentration of these organoarsenicals decreased with depth and is considered to be degraded
within 60 years of deposition. In addition, Hasegawa
(1997) found that the detritus on the surface of lake sedi-ment was consumed by the anaerobic respiration of bacte-ria. The methylarsenic species were degradation products and/or the results of in situ bacterial methylation and were decomposed to inorganic arsenic under anoxic conditions by facultative and obligate anaerobes.
It was also reported that a portion of the DMAAVwas
transformed in vitro to AsBet by some organisms (Kaise
Table 4
Results of different natural sample analysis (concentration in lg l1)
Water samples IC-ICP-MS ICP-MS
AsBet DMAAV AsIII MMAAV AsV Total
Drinking waters Well water 1 a a 0.22 ± 0.03 a 1.40 ± 0.15 1.62 ± 0.15 1.76 Well water 2 a a 0.44 ± 0.06 a 1.86 ± 0.19 2.30 ± 0.20 2.13 Well water 3 a a 0.87 ± 0.11 a 1.03 ± 0.12 1.90 ± 0.16 1.98 Surface water 1 b 0.12 ± 0.01 0.23 ± 0.03 a 1.79 ± 0.21 2.12 ± 0.21 1.97 Surface water 2 b b 0.11 ± 0.01 a 0.24 ± 0.03 0.35 ± 0.03 0.38 Natural mineral waters
Sample 1d a a a a 8.52 ± 0.19 8.52 ± 0.19 7.30 Sample 2d a a a a 0.32 ± 0.04 0.32 ± 0.04 0.32 Sample 3d a a 0.07 ± 0.01 a 0.09 ± 0.01 0.16 ± 0.01 0.21 Sample 4d a a a a 26.20 ± 0.76 26.20 ± 0.76 26.54 Sample 5e a a a a a a 0.25 Sample 6c a a a a 0.08 ± 0.01 0.08 ± 0.01 a Sample 7c a a a a a a a Sample 8d a a a a 7.59 ± 0.25 7.59 ± 0.25 7.40 Sample 9d a a a a 0.55 ± 0.06 0.55 ± 0.06 0.76 Sample 10d a a a a 3.57 ± 0.32 3.57 ± 0.32 3.39 a <LOD. b <LOQ. c
Belgian mineral natural water.
d
French natural mineral water.
e
et al., 1997). Indeed, organisms are transformed to partic-ulate or dissolved species after their death, and experimen-tal studies in vitro demonstrated that arsenobetaine contained in organisms degraded to inorganic forms via
DMAAVintermediates (Hanaoka et al., 1993, 1997). This
mechanism could be at the origin of the low levels of AsBet
and DMAAVfound in our study. This arsenic cycle shall
explain why we detected both methylated and inorganic species in surface water.
Depending on the geological origin of the natural min-eral water, the arsenic content was very variable from
one type of water to the other (Table 4). According to
Van Holderbeke et al. (1999), arsenic in such samples was exclusively in the arsenate form. Nevertheless, it was very interesting to notice that some natural mineral waters had a total arsenic content exceeding the 10 lg l1limit of the European Commission Directive 2003/40/CE (2003). The presence of arsenic at this concentration level should be associated with the geochemical environment. Indeed, occurrence of arsenic in natural water depends on the local geology, hydrology and geochemical characteristics of the aquifer. In our study, the natural mineral waters with the higher arsenic content were located in a volcanic region
in France.Thomas and Sniatechi (1995)reported that some
mineral or thermal water were exceeding the maximum drinking water limit for arsenic values, e.g. spring water from Puy du Doˆme in the volcanic region of Massif Central in France.
4. Conclusions
A method for arsenic speciation by IC-ICP-MS was developed which allowed the simultaneous separation of
five arsenic species (AsBet, DMAAV, AsIII, MMAAV and
AsV) in various types of water samples with LODs close
to 20 ng l1for each arsenic compounds. In each case the
total arsenic concentration obtained by ICP-MS was in good agreement with the sum of the arsenic species obtained by IC-ICP-MS.
Based on arsenic speciation in well and surface waters, arsenite and arsenate were the major molecules, while in
surface water, both DMAAVand AsBet were found. The
occurrence of these methylated forms was probably due to micro-organisms as in well and natural mineral waters, no methylated products were detected.
The concentration of arsenic in some natural mineral waters analysed in this study exceeded the European Direc-tive 2003/40/CE limit. The developed method is available to identify and quantify the arsenic species present in natural and drinking water at trace levels, which might be particularly important for the toxicity assessment in regions that may suffer of natural arsenic contamination. Acknowledgments
The authors are grateful to Mrs. Patricia Pierre and Mr. Le´on Ronkart for technical assistance and support during
experimentations and the preparation of the draft, and to Prof. Georges C. Lognay for critical remarks during the writing of the manuscript.
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